In data communication technology, the need often arises whereby a greater attenuation in certain stop band frequencies of a passband filter is required than what is achievable using a conventional passband filter. One such example is the filter used in the front end of a radio telephone. As is well known to persons skilled in the art in a radio telephone a received radio frequency signal is mixed, after the front end filter, which may be, for example, the receiver branch of a duplex filter, in a mixer with a signal from a local oscillator (LO) to provide an intermediate frequency (IF) signal. The frequency of the local oscillator signal is offset by the intermediate frequency from the frequency of the received signal. In a mixer the resultant output signals are the sum and difference of the local oscillator and received signal frequencies, with the undesirable frequencies produced being filtered out in an intermediate frequency filter so that only signals of the desired frequency remain. Thus, when employing a mixer no difference can be made between a desired received signal and a signal having the mirror (or image) frequency, the image frequency being that frequency which differs from the local oscillator (LO) frequency by the IF, but at the other side of the LO frequency from the received signal. As an example, let the local oscillator frequency be A MHz and the desired received signal frequency B MHz, this being greater than the A MHz. The intermediate frequency (IF) obtained as a result of the mixing is the difference between these frequencies, i.e. IF=(B-A) MHz. There is also the so-called image frequency which, as described above, is smaller than the LO frequency by the magnitude of the IF, so that if a signal having the image frequency C MHz is mixed with the LO signal, an IF signal having a frequency IF=(A-C) MHz results, which is the same as (B-A) MHz. This is illustrated in FIG. 2. The IF filter is not capable of distinguishing between the frequencies (B-A) and (A-C), although only the signal of frequency (B-A) is wanted. Because of this, the image frequency signal C has to be filtered in the front end filter before the mixer, so that it will not be coupled to the mixer, and only the signal of frequency B (which contains the required information) is shifted to the intermediate frequency.
This filtering is achieved using a bandpass filter, but unfortunately, the requirements for such a filter are mutually contradictory. The attenuation of the passband filter is required to be low at the desired signal frequency (frequency B) but it must be able to attenuate strongly the undesirable image-frequency signal (frequency C) usually located in the proximity of the 3 dB limit frequency of the filter. Widening the passband reduces the transmission losses of the filter while simultaneously reducing also the attenuation in the mirror frequency. These contradictory requirements have been solved by adding one or more additional transmission zeroes to the transfer function of the filter, the zeroes being located at the frequency of the undesirable signal (frequency C). Adding a transmission zero can be done by means of a separate parallel resonator, or using a so-called phasing technique within the filter.
This principle of adding transmission zeroes using the phasing technique is described in the patent No. U.S. Pat. No. 4,418,324 and it is summarized below with reference to the accompanying FIGS. 1 and 2. A passband filter includes four adjacent quarter wavelength resonators 1,2,3,4, one end of each resonator being grounded. The resonators 1,2,3,4 are strip-line resonators arranged interdigitally although it is obvious to a person skilled in the art that resonators of other types may be used. The coupling between the resonators is electromagnetic, coupling across-depending on the structure of the filter-air (in a helix resonator), an insulation plate (in microstrip and strip-fine resonators), or a ceramic plate (in a ceramic resonator), and the intensity of the coupling is dependent on the distance between the resonators. The input of a signal to the first resonator 1 and the output of the signal from the last resonator 4 can be carded out e.g. by tapping as is known to a person skilled in the art. As is also known to a person skilled in the art, each resonator 1,2,3,4 determines one pole in the transmission function so that a desired passband filter can be constructed by varying the structure. A first transmission zero of the transfer function is produced by coupling a conductive line or conductive channel between the open ends of two non-adjacent resonators 1 and 3, the transmission line or conductive channel comprising, a controllable capacitance 6, a transmission line 5, and a second controllable capacitance 7 coupled in series. A second transmission zero may be similarly produced by coupling a second conductive transmission line or conductive channel between the open ends of the other set of nonadjacent resonators 2 and 4. The second conductive transmission line or channel similarly consisting of a controllable capacitance 9, a transmission fine 8, and a second controllable capacitance 10 coupled in series. In this way a reverse-phased component is coupled to the resonator, and dependent on the amplitude a given additional attenuation can be provided in a given point of the frequency curve.
In the above-mentioned patent, interdigitally arranged resonator strips are located between two insulator plates, with the grounded surfaces being located on the other side of the plates from the resonator strips (i.e. a strip-line structure). On one of the grounded surfaces, the conductive channels are provided by transmission strips (produced by etching on the grounded surface), which have widened ends or pads arranged to be adjacent the open or non-grounded, ends of two non-adjacent resonators 1,2,3,4 located on the opposite side of the insulator plate. Each pad forms a parallel plate capacitor with the open ends of the resonators. By changing the sizes of the widened ends the capacitances can be changed and thus, the locations of the transmission zeroes can be separately and precisely selected as desired. The transmission zeroes may also be placed one on top of the other, whereby an extremely high attenuation can be produced for the frequency in the attenuation curve of the filter.
FIG. 2 shows graphically the impact of the addition of transmission zeroes. The broken line curve illustrates the frequency response of the filter when no transmission zeroes have been added. A signal B at the received frequency passes through the filter without becoming essentially attenuated, whereas a signal at mirror frequency C is not sufficiently attenuated. By adding at least one transmission zero in the mirror frequency C, the frequency can be attenuated further without exerting any influence on the attenuation of the pass frequency B proper, this being shown in curve d. The addition of a transmission zero slightly weakens the attenuation also at the upper end of the attenuation curve, but the drawback is fairly insignificant in the present application. A transmission zero may also be added above the frequency B when wishing to have a "recess" at this point of the attenuation curve.
The transmission line produces a reverse-phased component at a desired frequency in the attenuation curve, the amplitude determining the additional attenuation to be produced at that point. Hereby, a transmission zero point is generated at this point of the attenuation curve.
In practice, the supplier of the filter sets the location of the transmission zero by reducing the widened ends by means of a laser or by removing material, whereafter no further setting is done. The setting may, at least in certain practical designs, be accomplished by means of controllable capacitances.
The prior art methods of setting the transmission zero involve a variety of drawbacks. Firstly, the transmission zero is frequently selected in the manufacturing phase as described above, and setting it may turn out to be difficult as it requires the removal of material with laser or by grinding. Secondly, if one manages to produce the capacitors 6,7,9 and 10 so that the selecting of the transmission zero is possible after the manufacturing, the power travelling through the transmission line will cause problems with regard to the duration of the power of the adjusting capacitor. Such drawbacks may, in fact, be removed by abandoning the transmission lines, and by adding, instead, parallel resonators in the filter, though this will impair the Q value of the filter.